Estimating Turbulence Distribution over a Heterogeneous Path Using Time-Lapse Imagery from Dual Cameras

Knowledge of turbulence distribution along an experimental path can help in effective turbulence compensation and mitigation. Although scintillometers are traditionally used to measure the strength of turbulence, they provide a path-integrated measurement and have limited operational ranges. A technique to profile turbulence using time-lapse imagery of a distant target from spatially separated cameras is presented here. The method uses the turbulence induced differential motion between pairs of point features on a target, sensed at a single camera and between cameras to extract turbulence distribution along the path. The method is successfully demonstrated on a 511 m almost horizontal path going over half concrete and half grass. An array of Light-Emitting Diodes (LEDs) of non-uniform separation is imaged by a pair of cameras, and the extracted turbulence profiles are validated against measurements from 3D sonic anemometers placed along the path. A short-range experiment with a heat source to create local turbulence spike gives good results as well. Because the method is phase-based, it does not suffer from saturation issues and can potentially be applied over long ranges. Although in the present work, a cooperative target has been used, the technique can be used with non-cooperative targets. Application of the technique to images collected over slant paths with elevated targets can aid in understanding the altitude dependence of turbulence in the surface layer.

Differences in the Evolution of Pyrocumulonimbus and Volcanic Stratospheric Plumes as Observed by CATS and CALIOP Space-Based Lidars

Recent fire seasons have featured volcanic-sized injections of smoke aerosols into the stratosphere where they persist for many months. Unfortunately, the aging and transport of these aerosols are not well understood. Using space-based lidar, the vertical and spatial propagation of these aerosols can be tracked and inferences can be made as to their size and shape. In this study, space-based CATS and CALIOP lidar were used to track the evolution of the stratospheric aerosol plumes resulting from the 2019–2020 Australian bushfire and 2017 Pacific Northwest pyrocumulonimbus events and were compared to two volcanic events: Calbuco (2015) and Puyehue (2011). The pyrocumulonimbus and volcanic aerosol plumes evolved distinctly, with pyrocumulonimbus plumes rising upwards of 10 km after injection to altitudes of 30 km or more, compared to small to modest altitude increases in the volcanic plumes. We also show that layer-integrated depolarization ratios in these large pyrocumulonimbus plumes have a strong altitude dependence with more irregularly shaped particles in the higher altitude plumes, unlike the volcanic events studied.

Model Calculations on the Influence of Charged Mesospheric dust on the Incoherent Radar Spectrum

<p>Detection of charged dust in the spectrum of incoherent radars has previously been proposed and examined to some degree. These dust particles are of nanometer size and reside at mesospheric altitudes due to incoming ablating meteors. They are difficult to detect and thus their influence on atmospheric processes is hard to determine. Theoretical studies suggest that charged nanometer sized dust in the mesosphere can be successfully detected in the radar spectrum. However, current radar systems like EISCAT are not capable to distinguish adequately the dust signal from the main signal because the influence is small. We expect however, that the upcoming new EISCAT_3D radar will improve the observation conditions. We here present model calculations to examine the influence of the charged dust component on the radar signal, a so-called dusty plasma effect. Instead of the previously assumed one size dust component, we simulate the incoherent scatter spectrum including a large set of dust size bins. We show that different sizes, number density and charge of dust influence the signal in different ways, either causing a narrowing or broadening of the spectrum. Here the results are presented in a systematic way and specific conditions identified that provide the largest chance of dust detection in the signal. A simple charging model is used to model the most probable charge and altitude dependence to simulate realistic dust distributions that are then used as input to the radar spectrum model. These results can then be used to compare with actual radar measurements. Off which the new EISCAT_3D radar system, ready in 2022, might provide the adequate resolution for these requirements.</p>

Schumann Resonance on Titan : Huygens Observations Critically Re-Assessed and prospects for the Dragonfly Mission

<p>The Huygens probe to Titan in 2005 was the first planetary probe or lander to feature ELF electric field sensing and atmospheric conductivity measurements. The atmospheric electricity community showed great interest in the claimed detection of a Schumann resonance signal on another world (despite its unexpected dominant frequency of 36 Hz), and the planetary science community embraced an interpretation of the altitude dependence of the signal as evidence of a theoretically-anticipated internal water ocean beneath an ice crust many tens of km thick.</p><p>Quantitative scrutiny suggests that prospects of detecting a Schumann signal at Titan with the Huygens experiment were in fact very poor, due to short measurement time, a horizontal antenna orientation, a lack of lightning, and the likely presence of severe dynamical effects on the probe. Although the latter objections were considered, and arguments developed against them (notably the novel postulated Saturn-magnetospheric excitation of the resonance), we have re-examined the data in the light of a better understanding of the probe dynamics. The evolution of the 36Hz power shows a very strong correlation with accelerometer records of short-period motions of the probe under its small stabilizer parachute, suggesting that mechanical oscillations of the probe and/or the antenna booms were actually the cause. The ‘signal’ ramped up just as the probe accelerated from the much more quiescent main parachute, and ceased abruptly a couple of seconds after impact.</p><p>While the Huygens signal may therefore have been an artifact, this does not mean that a Schumann resonance does not occur on Titan. Most likely if it occurs, it may be very sporadic, responding to the infrequent rainstorms on Titan. A search for such signals should therefore be a long-duration monitoring exercise (not unlike listening for seismic events that could also probe Titan’s interior). The Dragonfly mission to Titan, recently selected for launch in 2026 with arrival planned in 2034 and over two years of surface operation, provides an opportunity to perform such monitoring.</p>

Climatic Effects of Hygroscopic Growth of Sulfate Aerosols in the Stratosphere

<p>Deliberate climate intervention by injection of sulfate aerosols in the stratosphere is a method proposed to counter anthropogenic climate warming. In such an injection scenario, an improved understanding of the microphysical and optical properties of the injected aerosols is important as these properties alter the radiative forcing and resulting climate. Here we analyze the effect of a specific microphysical property of sulfate aerosols in the stratosphere: hygroscopic growth – the tendency of aerosol particles to grow by accumulating water. In the NCAR CESM model, using idealized climate simulations, we find that, for a given mass, stratospheric sulfate aerosols cause more cooling when prescribed at the lower levels of the stratosphere because of increased hygroscopic growth of the aerosols due to larger relative humidity. The relative humidity in the stratosphere typically decreases rapidly with the increasing altitude. The larger relative humidity in the lower stratosphere causes an increase in the aerosol size through hygroscopic growth, which leads to a larger scattering efficiency. The increase in shortwave back-scattering due to the size change is found to be the primary factor contributing to the additional surface cooling as the aerosols are prescribed in the lower levels of the stratosphere. In our simulations, hygroscopic growth provides an additional cooling of 23% (0.7 K) when 20 Mt-SO4 of sulfate aerosols are prescribed at 100 hPa, relative to a non-hygroscopic simulation where hygroscopic growth is not allowed in the stratosphere. This additional cooling due to hygroscopic effect becomes weaker higher in the stratosphere where relative humidity is lower. Hygroscopic growth also leads to additional warming in the layers where the aerosols are prescribed due to an increase in near-IR shortwave absorption. This warming causes secondary effects such as a decrease in high clouds and an increase in stratospheric water vapor, which affects the effective radiative forcing. This altitude dependence of the cooling effects of hygroscopic growth is opposite to the altitude dependence of sedimentation effects;  while the hygroscopic effect produces larger cooling when aerosols reside in the lower stratosphere, the sedimentation effect produces less cooling when aerosols are injected into the lower stratosphere as the residence time becomes shorter.</p>

Defining aerosol layer height for UVAI interpretation using aerosol vertical distributions characterized by MERRA-2

Aerosol vertical distributions are important for aerosol radiative forcing assessments and atmospheric remote sensing research. From our perspective, the aerosol layer height (ALH) is one of the major concerns in quantifying aerosol absorption from the ultra-violet aerosol index (UVAI). The UVAI has a global daily record since 1978, whereas a corresponding ALH data set is still limited. In this paper, we attempted to construct such an ALH data set from aerosol extinction profiles provided by the MERRA-2 aerosol reanalysis, meanwhile we evaluated them, together with several satellite ALH products in terms of the UVAI sensitivity to ALH. In the first part of this paper, we derived ALHs from the MERRA-2 aerosol profiles by four definitions. Through the sensitivity studies, we found that the definition of top boundary aerosol layer height (Haert) is more robust to the changes in extinction profile properties than others. The spatial and temporal variation of Haert are also well associated with the major aerosol sources and the atmospheric dynamics. In the second part, we further evaluated the UVAI altitude dependence on the MERRA-2 ALH as well as several satellite ALH. Among all the satellite ALH products in this paper, the correlation between the TROPOMI oxygen (O2) A-band ALH and UVAI, and that between the GOME-2 absorbing aerosol layer height (AAH) and UVAI are in agreement with our a-priori knowledge that the altitude dependence of UVAI increases with aerosol loadings. The correlation between the MERRA-2 Haert and UVAI also matches well with what we found from observational data sets. This implies the top boundary of the aerosol layer derived from MERRA-2 can be an alternative in case there is no observational ALH data available for quantitively aerosol absorption from UVAI and other UVAI-related applications.

Modelling the Altitude Dependence of the Wet Path Delay for Coastal Altimetry Using 3-D Fields from ERA5

Wet path delay (WPD) for satellite altimetry has been provided from external sources, raising the need of converting this value between different altitudes. The only expression available for this purpose considers the same altitude reduction, irrespective of geographic location and time. The focus of this study is the modelling of the WPD altitude dependence, aiming at developing improved expressions. Using ERA5 pressure level fields (2010–2013), WPD vertical profiles were computed globally. At each location and for each vertical profile, an exponential function was fitted using least squares, determining the corresponding decay coefficient. The time evolution of these coefficients reveals regions where they are highly variable, making this modelling more difficult, and regions where an annual signal exists. The output of this modelling consists of a set of so-called University of Porto (UP) coefficients, dependent on geographic location and time. An assessment with ERA5 data (2014) shows that for the location where the Kouba coefficient results in a maximum Root Mean Square (RMS) error of 3.2 cm, using UP coefficients this value is 1.2 cm. Independent comparisons with WPD derived from Global Navigation Satellite Systems and radiosondes show that the use of UP coefficients instead of Kouba’s leads to a decrease in the RMS error larger than 1 cm.

Chlorine partitioning in the lowermost Arctic vortex during the cold winter 2015/2016

Activated chlorine compounds in the polar winter stratosphere drive catalytic cycles that deplete ozone and methane, whose abundances are highly relevant to the evolution of global climate. The present work introduces a novel dataset of in situ measurements of relevant chlorine species in the lowermost Arctic stratosphere from the aircraft mission POLSTRACC–GW-LCYCLE–SALSA during winter 2015/2016. The major stages of chemical evolution of the lower polar vortex are presented in a consistent series of high-resolution mass spectrometric observations of HCl and ClONO2. Simultaneous measurements of CFC-12 are used to derive total inorganic chlorine (Cly) and active chlorine (ClOx). The new data highlight an altitude dependence of the pathway for chlorine deactivation in the lowermost vortex with HCl dominating below the 380 K isentropic surface and ClONO2 prevailing above. Further, we show that the Chemical Lagrangian Model of the Stratosphere (CLaMS) is generally able to reproduce the chemical evolution of the lower polar vortex chlorine budget, except for a bias in HCl concentrations. The model is used to relate local measurements to the vortex-wide evolution. The results are aimed at fostering our understanding of the climate impact of chlorine chemistry, providing new observational data to complement satellite data and assess model performance in the climate-sensitive upper troposphere and lower stratosphere region.